Merpro's first chance came after 10 minutes following a Smith free kick Donaldson won a challenge with the keeper but his header went over the bar.
A gilt edged chance was created by Christie setting up Milne to break the Portcullis rearguard an
d run through on the keeper but his low shot was well saved.
Findlay, making his second deputising appearance in goal this season, then made a good save from a Portcullis free kick in 15 minutes.
Craig Allan made a good break from midfield and only a good tackle by Falkner prevented a goal scoring opportunity.
From the resultant corner kick Merpro opened their account. Craig Allan's cross was partly headed clear only to find Steven Donaldson waiting in the box and he sent a placed header into the net from 15 yards.
Portcullis then came back right back into the match with a couple of quickfire goals largely as a result of Merpro concentration lapses.
A long ball free kick fell for Twaddle to head home from 10 yards after he was allowed to run off his marker to make it 1-1 after 30 minutes. A few minutes later an innocuous long cross came into the box only for a mix up resulting in Findlay fumbling his catch leaving Twaddle with an easy opportunity to knock into the net after 33 minutes.
However Merpro maintained their focus and an equaliser was soon conjured with an excellent goal. Whitworth produced a sublime long diagonal pass over the defence for Craig Christie to run through on goal and fire in a low first time shot.
Merpro opened the second half brighter with Houston, Donaldson and Christie creating some opening chances with good play down the left.
Merpro had a couple of free kicks on the edge of the box but neither Christie or Whitworth were able to find the target.
Merpro were now dominating possession and Whitworth went close in 60 minutes.
Merpro finally went ahead, when a good move involving Milne and low curling cross from Donaldson left Sean Whitworth to score from 8 yards after his first effort was saved by the keeper.
Merpro went further ahead when Graham came short and flicked a pass behind the defence for Steven Donaldson to run through and drill in a low shot into the corner of the net from 15 yards.
Merpro created another opportunity when Archbold won a header to set up Milne but his effort went over the bar in 82 minutes.
Although not a vintage Merpro performance this was another win under their belts to take them safely into the next round .
Merpro: Findlay, Wilson (Duff), Houston, C Allan, Smith, G Allan, Brown, Whitworth (Archbold), Christie (Graham), Milne, Donaldson. Subs: Doyle, Stephen.
Match Sponsor : Nuffield Tractor Rep. On Saturday, Merpro are at home to Morgan FPs in the Ferrari's Shield 2nd Round kick-off 2pm.
